The following abbreviations are herewith defined, at least some of which are referred to within the following description of the state-of-the-art and the present invention.    AWG Arrayed Waveguide Grating    CO Central Office    OLT Optical Line Terminal    ONT Optical Network Terminal    ONU Optical Network Unit    OTDR Optical Time Domain Reflectometry    PIC Photonic Integrated Circuit    PLC Planar Lightwave Circuit    PON Passive Optical Network
A PON (passive optical network) is often employed as an access network or (from a different perspective) as the access portion of a larger communication network. Large communications networks generally have a high-capacity internal or core portion where data or information associated with, for example, television, telephone service, or Internet access is carried across great distances. The core network may also have the ability to interact with other networks to complete telephone calls, enable other two-way or multi-party communications, or request and receive content for delivery to individuals or business subscribers.
The access portion of a communications network, which may also be referred to as an access network, extends from the core or core portion of the network to individual subscribers, such as those associated with a residence or small business location. Access networks may be wireless access, such as a cellular telephone network, or fixed access, such as a PON or cable network. The access network typically though not necessarily ends at a demarcation point on or near the outside of a subscriber premises.
In a PON, as the name implies, optical fibers and interconnecting devices are used for most or all of the communication through the extent of the access network. While only recently it was relatively unusual for an individual residence to be served by an optical fiber, it is now common and may soon become nearly universally available. The basic components of a typical PON are shown in FIG. 1.
FIG. 1 is a simplified schematic diagram illustrating selected components of a typical PON 100 according to the existing art. ONTs (optical network terminals) 115a through 115n are devices typically found on the outside of subscribers' homes or other premises. As the ellipsis in FIG. 1 implies, there may be any number of such devices in a PON that are associated with a single optical splitter. The optical fibers connecting the splitter to the ONTs it serves are generally referred to as access (or “drop”) fibers. The optical splitter is typically located in a street cabinet or similar structure with many other optical splitters (not shown for clarity), each serving their own set of ONTs or other ONUs (optical network units). (An ONU may also be a terminal device that serves many subscribers, such as at an apartment building. The term ONT is usually applied to a single-subscriber device.)
In the exemplary PON 100 of FIG. 1, an OLT (optical line terminal) 105 interfaces with a core network (not necessarily using optical signals). In this capacity, OLT 105 forms the optical signals for transmission downstream to ONTs 115a through 115n along a feeder fiber to optical splitter 110. Optical splitter 110 is typically a passive device that simply distributes the signal received from OLT 105 to all the ONTs it serves. Each ONT is then responsible for selecting the portions of the transmitted signal that are intended for its subscriber and passes them along. Other portions of the transmitted signal are simply discarded.
Upstream transmissions from ONTs 115a through 115n are often transmitted in bursts according to a schedule provided to each ONT. In this way, none of the ONTs 115a through 115n send upstream transmissions at the same time. In most applications, upstream transmissions are less frequent than those in the downstream direction and so having to wait for an assigned time slot does not affect upstream performance too significantly. Upstream and downstream transmissions are often sent using different wavelengths of light so as not to interfere with each other.
During PON operation, it is often desirable to assess the integrity of some or all of the fibers, such as the feeder fiber and access fibers shown in FIG. 1. Fibers often extend for some distance and may not be readily accessible. When a fiber break occurs, it may be difficult to detect and localize. Even if a visible inspection is possible, not all breaks or other problems will be readily apparent. Current methods of inspection often focus on and are more accurate with regard to the feeder portion of the line. A solution is therefore needed that can efficiently test the access fibers for breaks or other degradation so that appropriate repairs may be quickly made.
Accordingly, there has been and still is a need to address the aforementioned shortcomings and other shortcomings associated with assessing the integrity of PON fiber optic lines, especially the access fibers. These needs and other needs are satisfied by the present invention.